A uniflow two-stroke engine that includes an exhaust port provided in an upper end part of the cylinder and a scavenging port provided in a lower side portion of the cylinder is known. The scavenging port is opened and closed by a side portion of the piston reciprocating in the cylinder. In this engine, combustion occurs when the piston is in the vicinity of the top dead center, and as the piston moves downward, the exhaust valve is opened and the expanded combustion gas (exhaust gas) is discharged through the exhaust port. At this time, in a typical two-stroke engine, the downward movement of the piston compresses the air-fuel mixture in the crank chamber while opening the scavenging port, so that the air-fuel mixture in the crank chamber flows into the cylinder via the scavenging port. Thereby, the combustion gas in the cylinder is pushed out through the exhaust port by the entering air-fuel mixture. At this time, if the layer of air-fuel mixture flowing into the cylinder and the layer of the combustion gas are not mixed with each other and a clear boundary therebetween is maintained, it is possible to discharge only the combustion gas via the exhaust port. However, part of the air-fuel mixture is mixed with the combustion gas or has a velocity higher than that of the combustion gas, so that the part of the air-fuel mixture is discharged through the exhaust port to the outside together with the combustion gas, which phenomenon is known as “blow-by.” The blow-by of the air-fuel mixture is not favorable in view of fuel consumption and environmental pollution.
To address such a problem, there is an engine having an air-fuel mixture separator disposed on a path passing the scavenging port (see JP5039790B, for example). In this engine, the air-fuel mixture is caused to pass through a centrifugal-type separator and separated into a fuel-rich air-fuel mixture and a fuel-lean air-fuel mixture, which are supplied to the cylinder via different passages. Thus, by using the fuel-lean air-fuel mixture to perform the scavenging, it is possible to decrease the concentration of fuel discharged through the exhaust port.
With regard to the uniflow two-stroke engines, in addition to the demand for suppression of the aforementioned blow-by, there is a demand for use of a compression ignition system to improve thermal efficiency. To perform stable compression ignition, it is necessary to maintain a high temperature of the air-fuel mixture supplied to the cylinder. With regard to the scavenging, it is preferred to discharge the combustion gas in the cylinder as much as possible from the point of view of volumetric efficiency (intake efficiency), but this would waste the energy of the combustion gas (exhaust gas), so that the temperature of the cylinder and the air-fuel mixture flowing into the cylinder is lowered, leading to unstable compression ignition.